With an air conditioner having a compressor for circulating refrigerant within a refrigerating cycle, and in which the compressor is coupled to an induction motor driven by a fixed frequency AC power source, the room temperature is controlled by turning on and off the compressor and hence the induction motor while comparing an actual room temperature with a setting temperature and making the difference therebetween zero. In contrast, with air conditioners known as inverter air conditioners, the room temperature is controlled more properly by regulating the air conditioner capability through variable speed running of the compressor, the variable speed running being conducted with the inverter and AC motor.
In both cases, the motors is turned off in order to stop the compressor. In this case, motor for fixed frequency type air conditioners are stopped while the motor is rotating at a relatively high rotational speed corresponding to the fixed frequency, e.g., 50 Hz or 60 Hz, whereas on the other hand the motor for inverter air conditioners are stopped while the motor is rotating at a relatively low rotational speed corresponding to a relatively low frequency.
The load of an air conditioner compressor or driving motor therefor varies greatly during one rotation, between a maximum torque immediately before discharge of the compressor, and a minimum torque at the start of suction of the compressor. Considerable vibration may be generated therefore depending on a shaft position when the compressor is stopped. For instance, in the case of fixed frequency window type air conditioners, upon turning off the motor power, the motor is stopped rapidly from a relatively high rotational speed corresponding to the commercial power source frequency, resulting in large vibrations which are undesirable from the standpoint of maintenance and noise control.
It can be thought that by detecting a shaft position, the motor is controlled to be stopped at a particular shaft position which allows minimum vibration. However, according to the related art, if a Hall effect device is used as a shaft position detecting element, it is difficult in practice to mount such a device on compressors, particularly of the closed type.